Inductively coupled plasmas work on the principal of producing an electric field in a body of gas via the magnetic fields produced by an oscillating current in the vicinity of the gas. When the fields induced in the gas are strong enough, the gas can break down and become ionized, resulting in what is commonly known as a glow discharge or plasma. Such plasmas have been used for a number of applications ranging from fluorescent lighting to plasma treatment of semiconductor wafers. In fluorescent lighting, a plasma induced within a lamp envelope generates many spectral components of light, the spectral components each occupying a narrow wavelength range and the spectral components occurring over a wide range of wavelengths depending on the gas in the lamp envelope. In many gases, a large portion of the spectral components can occur in the ultraviolet region. For the typical fluorescent lighting device, the ultraviolet light is converted to visible light by placing a phosphor coating on the lamp envelope. In semiconductor fabrication, plasmas provide the ions and other reactive gases used in semiconductor fabrication processes such as etching, resist stripping, passivation and deposition.
Traditionally, inductively coupled plasmas have been created by either wrapping a solenoidal coil around a glass or quartz tube containing a gas ("helical induction") or by placing such a coil within the volume of gas itself ("immersed induction"). In a typical approach, an RLC circuit created by the inductive coil and a matching circuit is tuned to resonance to develop high currents on the coil. An alternating magnetic field induced within the gas volume creates a conductive plasma discharge having characteristics like the secondary winding of a transformer, with a portion of the current through the discharge being converted to light. Lighting devices implemented using helical induction and others using immersed induction are described in U.S. Pat. No. 966,204, issued Aug. 2, 1910 to Hewitt.
A different approach to generating an inductively coupled plasma is described in U.S. Pat. No. 4,948,458, issued Aug. 14, 1990 to Ogle. Ogle teaches placing a planar spiral conductor proximate a gas-filled chamber. According to Ogle, a radiofrequency resonant current induced in the spiral conductor produces a planar magnetic field within the chamber. The resulting broad area plasma can be used to treat semiconductor wafers placed within the chamber. Hopwood, in "Review of Inductively Coupled Plasmas for Plasma Processing," published in Plasma Sources Science Technology in 1992, states that the diameter of a spiral-coupled inductively coupled plasma may be increased simply by increasing the diameter of the spiral coil. The result is a fairly uniform plasma which covers a large, planar area.
According to Ogle, the planar spiral inductive element should be placed on a planar dielectric surface through which it couples to a low pressure (0.1 mT to 5 Torr is claimed) gas. (According to Hopwood, the dielectric acts to reduce the effects of capacitive coupling between the coil and the plasma.) In the device taught by Ogle, the dielectric is a part of the chamber and, as such, it bears the load of the vacuum in separating the inductive coil from the low pressure gas. This becomes important when the plasma generator is scaled to larger coil sizes requiring larger, and hence thicker, dielectrics to bear the pressure differential.
Ogle teaches that the broad area plasma formed by driving the spiral coil in proximity to the chamber is useful for controlling etching and other ion-intensive semiconductor fabrication processes. At the same time, however, the planar spiral conductor is limited in its ability to adapt to different types of plasma generator applications and is still susceptible to irregularities in the plasma due to capacitive coupling between the spiral conductor and the gas. Finally, Ogle makes no provision for shielding the generator to prevent emissions of electromagnetic interference (EMI). There is a need in the art for a method of generating a uniform broad area plasma which is both flexible, adaptable to a variety of conditions and which addresses the above concerns regarding capacitive coupling and EMI.